Optical fiber maximizing residual mechanical stress

ABSTRACT

An optical fiber for maximizing residual mechanical stress and an optical fiber grating fabricating method using the optical fiber are provided. The optical fiber includes a core formed of silica, for propagating light, and a cladding formed of boron-doped silica, surrounding the core. Alternatively, the optical fiber includes a core formed of phosphorous-doped silica and a cladding formed of silica, surrounding the core.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 09/521,433 filed on Mar. 8, 2000 now U.S. Pat. No. 6,568,220.This related application is relied on and incorporated herein byreferences in its entirety.

CLAIM OF PRIORITY

This application claims priority to an application entitled “OpticalFiber In Which Residual Mechanical Stress Is Maximized and Method ForFabricating Fiber Gratings Using the Same” filed in the KoreanIndustrial Property Office on Mar. 11, 1999 and assigned Serial No.99-8080, the contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to an optical fiber whichmaximizes residual mechanical stresses and an optical fiber gratingfabrication method using the optical fiber, and in particular, to anoptical fiber with mechanical stresses maximized to write optical fibergratings therein and an optical fiber grating fabrication method usingthe optical fiber.

2. Description of the Related Art

An optical signal filter plays a significant role in improving theperformance of an optical communication system. Filters having opticalfiber gratings have recently attracted much interest since they can befabricated in optical fibers and no other external controlling devicesare needed. Optical fiber grating filters find wide application inoptical communications and optical sensors due to the advantages of lowloss and low cost. Optical fiber gratings are largely categorized intoBragg gratings (reflective or short-period gratings) and long-periodgratings (transmission gratings) according to their periods ofrefractive index changes in an optical fiber core.

The long-period fiber gratings are based on the principle that a greatchange in a refractive index occurs when irradiating an optical fiberwith a UV (UltraViolet) laser beam. An amplitude mask is usually used inwriting gratings in an optical fiber core and the photosensitivity ofthe optical fiber can be increased by loading the optical fiber with H₂.A conventional long-period optical fiber is fabricated in an opticalfiber having a germanium-doped core. Since gratings are writtenutilizing the photosensitiveness of the optical fiber, they cannot beformed in a non-photosensitive optical fiber by the conventionaltechnology. Another problem is that hydrogen treatment is required toincrease the photosensitiveness of an optical fiber.

Meanwhile, residual stresses has been used instead of photosensitivity.Residual stresses are divided into thermal stress and mechanical stress.The former is caused by the mismatch in the coefficients of expansioncoefficients between layers, while the latter is produced by differentviscosities between the layers, closely related with tensile force. Thethermal stress is not proportional to tensile force and its occurrencesare negligible. Thus, the mechanical stress is the dominant residualstress. To maximize the mechanical stress, a good choice of dopingmaterials for a core and a cladding is very important because theviscosities of the core and cladding vary with doping materials.

Examples of optical fiber compositions and optical fiber fabricationmethods of the contemporary art are given in the following U.S. patents.U.S. Pat. No. 4,426,129, to Matsumura et al., entitled OPTICAL FIBER ANDMETHOD OF PRODUCING THE SAME, describes an optical fiber including ajacketing layer, a cladding layer containing B₂O₃ as a dopant, and acore layer having a refractive index higher than the cladding layer.

U.S. Pat. No. 4,406,517, to Olshansky, entitled OPTICAL WAVEGUIDE HAVINGOPTIMAL INDEX PROFILE FOR MULTICOMPONENT NONLINEAR GLASS, describes anoptical fiber having a multimode core which has silica doped with GeO₂at the center and silica doped with B₂O₃ away from the center.

U.S. Pat. No. 4,447,125, to Lazay et al., entitled LOW DISPERSION SINGLEMODE FIBER, describes an optical fiber with germanium doped in the coreand fluorine doped in the cladding.

U.S. Pat. No. 4,410,345, to Usui et al., entitled METHOD OF PRODUCINGOPTICAL WAVEGUIDE, describes an optical fiber having a cladding made ofB₂O₃-silica glass.

U.S. Pat. No. 4,616,901, to MacChesney et al., entitled DOPED OPTICALFIBER, describes an optical fiber having a silica core doped with P₂O₅,and a cladding which may be pure or doped silica.

U.S. Pat. No. 4,810,276, to Gilliland, entitled FORMING OPTICAL FIBERHAVING ABRUPT INDEX CHANGE, discusses optical fibers having fluorine orboron-doped silica claddings, with germanium present in the core.

U.S. Pat. No. 5,694,502, to Byron, entitled BRAGG GRATINGS INWAVEGUIDES, describes a method of generating a Bragg reflective gratingin a photosensitive optical waveguide using a fringe pattern ofelectromagnetic radiation, and additionally employing heating by a CO₂laser in the region where the grating is being formed. This heating isto enhance the photosensitivity of the fiber.

U.S. Pat. No. 6,009,222, to Dong et al., entitled OPTICAL FIBRE ANDOPTICAL FIBRE GRATING, describes an optical fiber having agermanium-doped core and a boron-doped cladding. The patent furtherdiscusses a Bragg grating made in the fiber.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide animproved optical fiber for fabrication of optical fiber gratings.

Another object of the invention is to provide an optical fiber withenhanced mechanical stress.

Yet another object of the invention is to provide an optical fiber forfabrication of an optical fiber grating which does not require hydrogentreatment before writing a grating.

A still further object of the invention is to provide an improved methodfor fabricating an optical fiber grating.

These and other objects are achieved by providing an optical fiber formaximizing residual mechanical stress and an optical fiber gratingfabricating method using an optical fiber in which a core or a claddingis doped with a residual mechanical stress maximizing material. Theoptical fiber includes a core containing silica, for propagating light,and a cladding containing boron-doped silica, surrounding the core.According to another aspect of the present invention, the optical fiberincludes a core formed of phosphorous-doped silica and a cladding formedof silica, surrounding the core.

In the optical fiber grating fabricating method, an optical fiberpreform is formed to include a cladding formed of boron-doped silica anda core formed of silica, an optical fiber is drawn from the preform byapplying a predetermined tensile force to the preform, and gratings areformed in the optical fiber by annealing predetermined periodicalportions of the drawn optical fiber and thus relieving residual stressesof the optical fiber.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention, and may of the attendantadvantages, thereof, will be readily apparent as the same becomes betterunderstood by reference to the following detailed description whenconsidered in conjunction with the accompanying drawings in which likereference symbols indicate the same or similar components, wherein:

FIG. 1 is a graph showing the relationship between doping materials andviscosity;

FIGS. 2A and 2B illustrate a relative refractive index profile andviscosity profile of an optical fiber preform having a P₂O₅—SiO₂ coreand an F—SiO₂ cladding, respectively;

FIG. 3 illustrates viscosity versus relative refractive indexdifferences of the fiber preform of FIGS. 2A and 2B;

FIGS. 4A and 4B illustrate profiles of optical fibers according to thepresent invention;

FIG. 5 illustrates a long-period optical fiber grating fabricatingdevice using a CO₂ laser;

FIG. 6 is a graph showing the transmission spectrum of long-periodoptical fiber gratings formed in a silica core/boron-doped cladding DIC(Depressed Inner Cladding) optical fiber;

FIGS. 7 and 8 illustrate the transmission spectra of a long-periodoptical fiber grating formed in a P-doped core/matching cladding opticalfiber with D/d=4 and 8, respectively;

FIG. 9 illustrates a long-period optical fiber grating fabricatingdevice using an electric arc; and

FIG. 10 is a graph showing the transmission spectra of a long-periodoptical fiber grating formed in a P-doped core/matching cladding opticalfiber with D/d=8 using an electric arc and a CO₂ laser.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of the present invention will be describedhereinbelow with reference to the accompanying drawings. In thefollowing description, well-known functions or constructions are notdescribed in detail since they would obscure the invention inunnecessary detail.

Long-period optical fiber gratings can be formed utilizing residualstresses in an optical fiber with a pure silica core. Due to aphotoelastic effect, tensile force-incurred residual stresses reduce therefractive index of the core. The refractive index returns to itsoriginal level by partially relieving the residual stresses by a CO₂laser or an electric arc periodically. Then, the gratings are formed.

As to residual stresses in a step index optical fiber, the step indexoptical fiber includes a core and a cladding each formed of a differentmaterial and thus showing a different characteristic. When the opticalfiber is drawn with tensile load and high heat energy, each of the coreand cladding layers shifts from a solid state to a liquid state at adifferent transition temperature and is subject to tensile orcompressive stress by heat in each process. A different type of stressis mechanically induced when the optical fiber is cooled at roomtemperature. These two kinds of residual stresses remain in the opticalfiber.

Therefore, the residual stresses include thermal stress and mechanicalstress. The former is caused by the difference between the thermalexpansion coefficients of layers, whereas the latter occurs due to thedifference between the viscosities of the layers and has much to do withtensile force.

As to thermal stress, the core and cladding of an optical fiber areformed of different materials and thus have different thermal expansioncoefficients at different glass transition temperatures. It is knownthat the thermal expansion coefficient of each layer in the liquid stateis three times larger than that of the layer in the solid state. Hence,the optical fiber exhibits a very complicated thermal stress profile.But the thermal stress is less significant than mechanical stress as aresidual stress and is not easily controlled in a drawing process.

Mechanical stress, caused by the difference of viscosity between a coreand a cladding, is proportional to tensile force. Accordingly, residualstresses can be determined by controlling tensile force.

The mechanical stress generation mechanism can be considered in twostages: application of tensile force at high temperature; and release ofthe tensile force and decrease to room temperature.

In the first stage, a transformation rate based on elasticity andinelasticity is changed depending on the viscosity, surface area, andtensile force of each layer when an optical fiber is drawn at hightemperature. The relationship between tensile force F and stress at amelted preform portion can be expressed as $\begin{matrix}\begin{matrix}{F = {{3\eta_{1}A_{1}\frac{B_{V}}{B_{Z}}} + {3\eta_{2}A_{2}\frac{B_{V}}{B_{Z}}}}} \\{= {3\eta_{1}A_{1}\frac{B_{V}}{B_{Z}}\left( {1 + \frac{\eta_{2}A_{2}}{\eta_{1}A_{1}}} \right)}} \\{= {3\eta_{2}A_{2}\frac{B_{V}}{B_{Z}}\left( {1 + \frac{\eta_{1}A_{1}}{\eta_{2}A_{2}}} \right)}} \\{= {3\eta_{1}A_{1}\frac{B_{V}}{B_{Z}}G_{1}^{- 1}}} \\{= {3\eta_{2}A_{2}\frac{B_{V}}{B_{Z}}G_{2}^{- 1}}}\end{matrix} & \text{[Equation 1]}\end{matrix}$where η is viscosity, A is surface area, B is an elasticity coefficient,V is the shift rate of a melted preform portion, Z is the length of anoptical fiber in an axial direction, and subscripts 1 and 2 indicate acore and a cladding, respectively.

When the radius of the melted preform portion reaches that of theoptical fiber at a softening temperature, an initial stress iscalculated by $\begin{matrix}{{\sigma_{2}^{i} = {{3\eta_{2}\frac{B_{V}}{B_{Z}}} = {\frac{F}{A_{2}}G_{1}}}}{\sigma_{1}^{i} = {{3\eta_{1}\frac{B_{V}}{B_{Z}}} = {\frac{F}{A_{1}}G_{1}}}}} & \text{[Equation 2]}\end{matrix}$and initial and final elastic transformation rates ε₁ ^(i) and ε₂ ^(i)are determined by $\begin{matrix}{{ɛ_{1}^{i} = {\frac{F}{A_{1}E_{1}}G_{1}}}{ɛ_{2}^{i} = {\frac{F}{A_{2}E_{2}}G_{2}}}} & \text{[Equation 3]}\end{matrix}$

The initial elastic transformation rate lasts until the tensile force isreleased at room temperature. When the initial state transitions to afinal state, the optical fiber immediately recovers to a stable state, atransformation length involving the elastic transformation of each layeris the same in both the initial and final states, and the sum ofstresses imposed on each layer is 0. That is, $\begin{matrix}{{{ɛ_{1}^{f} - ɛ_{1}^{i}} = {ɛ_{2}^{f} - ɛ_{2}^{i}}}{{{A_{1}E_{1}ɛ_{1}^{f}} + {A_{2}E_{2}ɛ_{2}^{f}}} = 0}} & \text{[Equation 4]}\end{matrix}$

Then $\begin{matrix}{ɛ_{1}^{f} = {{{- \frac{A_{2}E_{2}}{A_{1}E_{1}}}ɛ_{2}^{f}} = {{{- \frac{A_{2}E_{2}}{A_{1}E_{1}}}\text{(}ɛ_{1}^{f}} - ɛ_{1}^{i} + {ɛ_{2}^{i}\text{)}}}}} & \text{[Equation 5]}\end{matrix}$

Therefore, the transformation of the core in the final state isexpressed as $\begin{matrix}{ɛ_{1}^{f} = \quad{{\frac{A_{2}E_{2}}{{A_{1}E_{1}} + {A_{2}E_{2}}}\left( {ɛ_{1}^{i} + ɛ_{2}^{i}} \right)}\quad = {{{\frac{F}{A_{1}E_{1}}A_{2}E_{2}G_{1}} - \frac{A_{1}E_{1}G_{2}}{{A_{1}E_{1}} + {A_{2}E_{2}}}}\quad = {\frac{F}{A_{1}E_{1}}g}}}} & \text{[Equation 6]}\end{matrix}$and the residual stress of the core in the final state is$\begin{matrix}{\sigma_{1}^{f} = {{E_{1}ɛ_{1}^{f}} = {\frac{F}{A_{1}}g}}} & \text{[Equation 7]}\end{matrix}$

Similarly, the transformation and stress of the cladding can be obtainedby $\begin{matrix}{{ɛ_{2}^{f} = {\frac{F}{A_{1}E_{1}}g}}{\sigma_{2}^{f} = {\frac{F}{A_{2}}g}}} & \text{[Equation 8]}\end{matrix}$

Residual stresses induced thermally and mechanically in an optical fiberare utilized for fabrication of long-period optical fiber gratings.Thermal stress does not involve a tensile force effect and causes anegligibly small change in refractive index relative to mechanicalstress. On the other hand, since mechanical stress is proportional totensile force, it can be relieved by heating the optical fiber with aCO₂ laser beam with tensile force released. Different refractive indexesare shown in a stress-relieved portion and a stress-remaining portionwhich alternate periodically in the optical fiber. A photoelastic effectwith respect to a stress-incurred reflectance change is computed by$\begin{matrix}{{{\Delta\quad n_{r}} = {{C_{a}\sigma_{r}} + {C_{b}\left( {\sigma_{\theta} + \sigma_{z}} \right)}}}{{\Delta\quad n_{\theta}} = {{C_{a}\sigma_{\theta}} + {C_{b}\left( {\sigma_{2} + \sigma_{r}} \right)}}}{{\Delta\quad n_{z}} = {{C_{a}\sigma_{z}} + {C_{b}\left( {\sigma_{r} + \sigma_{\theta}} \right)}}}} & \text{[Equation 9]}\end{matrix}$where C_(a) and C_(b) are photoelasticity coefficients of SiO₂, Δn is arefractive index variation, σ is stress, and r, θ, and z are radius,angle, and axial length, respectively. A refractive index in a radialdirection is significant to an optical signal propagated through anoptical fiber. Stress in an axial direction and a refractive indexvariation in a radial direction in a single-mode optical fiber are$\begin{matrix}{{\sigma_{z} = {\sigma_{1r}\left( {{K\quad\sigma_{r}},\sigma_{\theta}} \right)}}{\Delta_{n\quad r} = {C_{b}\sigma_{1r}}}} & \text{[Equation 10]}\end{matrix}$

Because C_(b) is −4.2×10⁻¹²Pa⁻¹, n₄ is a negative value with respect toextensibility σ_(lr). This implies that tensile stress decreases andcompressive stresses increases a refractive index. Accordingly, anoptical fiber should be drawn with a high tensile force, that is, at ahigher winding rate at a lower temperature than normal drawingconditions, for fabrication of long-period optical fiber gratings.

Now, there will be a description of doping materials of silica forlong-period optical fiber gratings.

An optical fiber is formed by doping silica with germanium, fluorine,phosphorous, boron, and so on. GeO₂ and P₂O₅ increase the refractiveindex of silica, whereas F and B₂O₃ decrease it. P₂O₅ and B₂O₃ are usedto decrease a process temperature and viscosity.

As GeO₂, P₂O₅, and B₂O₃ increase in mol %, the thermal expansioncoefficient of silica doped with them increases. The thermal expansioncoefficient of silica doped with F decreases with higher mol % of F. Theviscosity of silica glass reaches the highest limit if it is pure silicaand decreases if silica is doped with some material.

FIG. 1 is a graph showing doping materials versus viscosity. A viscositysensitivity coefficient with respect to a variation in the concentrationof a small amount of doping material is $\begin{matrix}{\quad{{K_{F} = \frac{B\quad{\log\lbrack\eta\rbrack}}{B\lbrack F\rbrack}}{K_{{GeO}_{2}} = {\frac{B\quad{\log\lbrack\eta\rbrack}}{B\left\lbrack {GeO}_{2} \right\rbrack}\left( {{\log\left\lbrack {PaE}_{s} \right\rbrack}/\%} \right)}}}} & \text{[Equation 11]}\end{matrix}$

Then, the log viscosities of silica doped with F and silica doped withGeO₂ are

 log[η]=K _(O) +K _(F) [F]  [Equation 12]log[η]=K _(O) +K _(GeO) ₂ [GeO ₂]  [Equation 13]

Here, relative refractive index differences of silica when it is dopedwith F and GeO₂ are a negative value and a positive value, respectively.They are known to be −0.5 and 1.5, respectively.

The variation of log viscosity in silica glass doped with F and GeO₂ isthe linear sum of variations in the log viscosity induced by individualF-doping and GeO₂-doping. Therefore, the log viscosity of silica glassdoped with F and GeO₂ islog[η]=K_(O) +K _(F) [F]+K _(GeO) ₂ [GeO ₂]

FIGS. 2A and 2B illustrate the relative refractive index profile andviscosity profile of an optical fiber preform having a P₂O₅—SiO₂ coreand an F—SiO₂ cladding, respectively. FIG. 3 illustrates viscosityversus relative refractive index differences shown in FIGS. 2A and 2B.

The viscosity sensitivity coefficient of P₂O₅ ranges between 15 and 23,expressed as Equation 15. That is, the viscosity sensitivity coefficientof P₂O₅ is greater than K_(F) because P₂O₅ and B₂O₃ drop a processtemperature and remarkably increase the viscosity of the glass.$K_{P_{2}O_{5}} = {\frac{B\quad{\log\lbrack\eta\rbrack}}{B\left\lbrack {P_{2}O_{5}} \right\rbrack}\left( {{\log\lbrack{PaEs}\rbrack}/\%} \right)}$

Accordingly, the viscosity and process temperature of silica can befurther dropped by doping silica with P₂O₅, relative to doping silicawith F or GeO₂. Meanwhile, it is noted from FIG. 1 that the sign ofK_(B203) is positive due to the negative value of a refractive indexvariation induced by B₂O₃ doping.

Viscosity is closely related with glass transition temperature Tg, thetemperature at which glass changes from a liquid state to a solid state.As the viscosity of glass decreases, its glass transition temperature isdropped. This is because the glass transition temperature occurs at aviscosity of about 10^(12.6)Pa·s.

FIGS. 4A and 4B illustrate the profiles of optical fibers according tothe present invention. The optical fibers are so constituted thatresidual mechanical stresses are maximized for forming long-periodoptical fiber gratings. After an optical fiber is drawn, tensile forceis applied on a layer with a relatively high viscosity and compressiveforce on a layer with a relatively low viscosity. Then, mechanicalstress remains in the optical fiber.

In FIGS. 4A and 4B, dotted lines indicate the profiles of optical fibersfree of mechanical stress, and solid lines the profiles of opticalfibers having mechanical stress.

An optical fiber shown in FIG. 4A has a core 41 formed of silica and acladding 42 surrounding the core 41, formed of silica doped with boron.A tube 43 covering the cladding 42 is formed of silica and has a higherviscosity than the cladding 42. Here, due to boron-doping, (that is,B₂O₃-doping) the viscosity of the cladding 42 is relatively very low,lower than even that of the core 41. The core 41 can be doped withgermanium to increase its refractive index or the cladding 42 canfurther be doped with fluorine to decrease its refractive index.

An optical fiber shown in FIG. 4B has a core 46 formed of silica dopedwith phosphorous, a cladding 47 surrounding the core 46, formed ofsilica, and a tube 48 covering the cladding 47, formed of silica. Thetube 48 shows a higher viscosity than the cladding 47. Due tophosphorous doping (that is, P₂O₅ doping), the viscosity of the core 46becomes lower and its refractive index is higher than that of thecladding 47. The core 46 can further be doped with germanium to increaseits refractive index or the cladding 47 with fluorine to decrease itsrefractive index.

Now there will be given a description of a method of forming long-periodoptical fiber gratings in an optical fiber with mechanical stressinduced therein using a CO₂ laser or an electric arc.

FIG. 5 illustrates a long-period fiber grating forming device using aCO₂ laser according to the present invention. The long-period fibergrating forming device is comprised of a CO₂ laser system 51, areflective mirror 52, a lens 53, a shelf 54, and a controlling computer55. The CO₂ laser system 51 includes a laser head, a power module, aremote controller, and a connection cable 51 a. The laser system 51emits a CO₂ laser beam in pulses so that a user can adjust the intensityand power of the emitted laser beam. The pulse width and period can becontrolled by the remote controller or a pulse generator connected tothe remote controller. The reflective mirror 52 is plated with gold andused for controlling a beam path. The lens 53 is fabricated of ZnSe, forfocusing a laser beam with an appropriate width.

The shelf 54 including an optical fiber fixture is moved by ahigh-resolution stepping motor 54 b controlled by the computer 55through an interface bus, such as a GPIB (General Purpose Interface Bus)54 a. A white light source and a spectrum analyzer can be furtherprovided to observe the transmission spectrum of long-period fibergratings during fabrication of the long-period fiber gratings.

FIG. 6 is a graph showing transmission spectrum characteristics oflong-period fiber gratings formed in a silica core/boron-doped DIGoptical fiber. In FIG. 6, a dotted line indicates transmission intensityversus wavelength of an optical fiber drawn with a 10 g tensile force,and a solid line indicates transmission intensity versus wavelength ofan optical fiber drawn with a 220 g tensile force. Here, the gratingperiod is 500 μm, the power of an output beam is 18W, an exposure timeis 0.2 sec, and the width of a beam is 200 μm. Thus, the energy densityis 1.7 J/mm². The fiber is exposed for one exposure time, the fiber ismoved by one beam-width, and then exposed again.

Referring to FIG. 6, the optical fiber drawn with the 10-g tensile forceshows no filtering effect, whereas the optical fiber drawn with the220-g tensile force exhibits a band rejection filter characteristicswith a peak of 20 dB at 1000 nm and a bandwidth of 25 nm. A changedrefractive index of the stressed regions long-period fiber gratings isreturned to its original refractive index by relieving residual stressesin portions of an optical fiber exposed to a beam periodically.

FIGS. 7 and 8 illustrate the transmission spectra of long-period fibergratings formed in a P-doped core/matching cladding optical fiber,respectively with D/d=4 and 8 (D is the radius of a cladding from thecenter of the core and d is the radius of the core). In FIGS. 7 and 8,dotted lines indicate H₂ loaded cases and solid lines H₂ unloaded cases.Long-period fiber gratings are formed under the conditions that thegrating period is 500 μm, the grating length is 2 μm, the power of anoutput beam is 12W, an exposure time is 0.2 sec, and the width of a beamis 377 μm. Therefore, the energy density is 1.2 J/mm².

As expected from the residual stress calculation, long-period fibergratings written in P-doped core/matching cladding optical fibers atD/d=4 and 8, respectively show a weaker filtering effect than in anoptical fiber with a core layer to which tensile force is applied.However, the filtering effect can be improved by increasing the gratinglength or loading the optical fiber with H₂. While a writing sensitivityis increased by H₂ loading at 100° C. at 100 bar for 72 hours, a pitchshifts toward a long wavelength.

FIG. 9 illustrates a device for forming long-period fiber gratings usingan electric arc. A pair of electrodes 91 adjust a discharge voltage. Anoptical fiber 93 is placed on a V-groove 92 and moves by a predetermineddistance periodically in a direction indicated by an arrow 94.

FIG. 10 illustrates the transmission spectra of long-period fibergratings when they are formed in P-doped core/matching cladding opticalfibers with D/d=8 by an electric arc and a CO₂ laser, respectively. Incomparison, coupling peaks appear almost at the same position and theefficiency of the electric arc is inferior to that of the CO₂ laser at acoupling mode. This is because a discharge time cannot be adjusted andan annealed boundary is not clear in the electric arc method.

More residual mechanical stresses are induced as the difference ofviscosity between layers of an optical fiber is wider and a greatertensile force is applied. Thus, refractive index changes to a greatextent and a long-period fiber grating effect are enhanced. Therefore,the present invention enables formation of long-period fiber gratingswith improved performance by adding P or B which remarkably reducesviscosity in an optical fiber.

As described above, residual mechanical stresses are maximized by addingP which drops viscosity markedly in a core or a cladding of an opticalfiber according to the present invention. Thus, long-period fibergratings can be written in the optical fiber more effectively.

While the invention has been shown and described with reference tocertain preferred embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims.

1. An optical fiber, comprising: a core comprising silica; and acladding surrounding the core said cladding comprising silica doped withB₂O₃, and said cladding having a value of refractive index lower thanthat of said core; said optical fiber having a first region and a secondregion formed periodically in a longitudinal direction of said firstregion, said second region having less residual stress than said firstregion, wherein a refractive index of said core in said second region ishigher than a refractive index of said core in said first region.
 2. Theoptical fiber of claim 1, said core further comprising germanium, forincreasing the refractive index of the core.
 3. The optical fiber ofclaim 1, said cladding further comprising fluorine, for decreasing therefractive index of the cladding.
 4. The optical fiber of claim 1,further comprising: a tube surrounding the cladding, said tubecomprising silica and having a higher viscosity than said cladding. 5.The optical fiber of claim 1, said second region being formed byspatially periodic heating of the optical fiber.
 6. An optical fiber,comprising: a core comprising silica doped with P₂O₅; and a claddingsurrounding the core, said cladding comprising silica, and said claddinghaving a value of refractive index lower than that of said core; saidoptical fiber having a first region and a second region formedperiodically in a longitudinal direction of said first region, saidsecond region having less residual stress than said first region,wherein a refractive index of said core in said second region is lowerthan a refractive index of said core in said first region.
 7. Theoptical fiber of claim 6, said cladding further comprising fluorine. 8.The optical fiber of claim 6, further comprising: a tube surrounding thecladding, said tube comprising silica and having a higher viscosity thansaid cladding.
 9. The optical fiber of claim 6, said second region beingformed by spatially periodic heating of the optical fiber.
 10. Theoptical fiber of claim 6, further comprised of: said cladding consistingessentially of silica; and said core consisting essentially of silicaand P₂O₅.
 11. The optical fiber of claim 6, further comprised of: saidcladding consisting essentially of silica and fluorine; and said coreconsisting essentially of silica and P₂O₅.
 12. The optical fiber ofclaim 6, further comprised of: said core consisting essentially ofsilica doped with P₂O₅ and germanium; and said cladding consistingessentially of silica doped with fluorine.
 13. An optical fiber grating,comprising: a first region comprising a first core and a first cladding,said first core comprising silica doped with phosphorous, said firstcladding surrounding said first core, said first cladding comprisingsilica, wherein said first region of said optical fiber is drawn undertensile force to stress said first core and said first cladding; and agrating formed periodically in a longitudinal direction of said firstregion, said grating comprising a second core and a second cladding,said second core comprising silica doped with phosphorous, said secondcladding surrounding said second core, said grating of said opticalfiber having less residual stress than said first region of said opticalfiber such that a refractive index of said first core is higher than arefractive index of said second core.
 14. The optical fiber grating ofclaim 13, said first cladding and said second cladding furthercomprising fluorine.
 15. The optical fiber grating of claim 13, furthercomprising: a tube surrounding said first cladding and said secondcladding, said tube comprising silica.
 16. The optical fiber grating ofclaim 1, wherein a period of said second region is 500 μm, and a lengthof said second region is 2 cm.
 17. The optical fiber grating of claim 1,wherein said core consists essentially of silica, and said claddingconsists essentially of silica doped with B₂O₃.
 18. The optical fibergrating of claim 1, wherein said cladding consists essentially of silicadoped with fluorine, and said core consists essentially of silica dopedwith B₂O₃.
 19. The optical fiber grating of claim 6, wherein a length ofsaid second region is more than 2 cm.
 20. The optical fiber grating ofclaim 6, wherein said optical fiber is loaded with H₂.